The invention relates to rotary printing machines, and is particularly, but not exclusively, applicable to offset printing machines of the kind having three main cylinders, namely a master cylinder, a blanket cylinder and an impression cylinder, which latter cylinder may be referred to as a paper cylinder; however, the invention could also be applied to for instance a direct offset printing machine, printing direct from the master directly onto the paper, or to a letter press.
In a conventional offset printing machine there are three main cylinders, a master cylinder which holds the master from which copies are to be made, a blanket cylinder which has a rubber blanket which receives an impression from the master cylinder for transfer to the paper, and an impression cylinder which holds a sheet of paper for receipt of the transferred impression from the blanket.
In such a conventional machine, the impression cylinder is provided with two or more grippers spaced in line across the cylinder and which have jaws which open at the appropriate moment to receive and grip the leading edge of a piece of paper carried forward and into contact with the grippers at the required instant in the cycle of operation of the machine.
It is a disadvantage of such a conventional machine that in order to change the position in which, in a longitudinal direction, the impression is received upon each piece of paper, it is necessary to stop the machine and to adjust the position of the grippers relative to the impression on the blanket, which is time consuming and which may result in adjustment in several steps being necessary before a satisfactory positioning of the impression is obtained. Particularly is this so when a subsequent impression has to be precisely positioned relative to a previous impression.
The object of the invention is to provide a rotary printing machine having means for varying the position of an impression on the sheets of paper while the machine is in operation.